Mid 80's Regan signed a bill preventing the legal registration of any more automatic weapons. Those already registered were still legal...with the right paperwork...but nothing new can be added to the list and the AA12 did not exist so I do not think it's possible to legally obtain one.

On the up side, if the Ass Hat in Chief gets his way and bans guns...the black market will explode so you should be able to find your AA12 for sale in the nearest dark alley...